The curse creates a barrier around a certain area, preventing anyone from passing through it and throwing them backwards on contact. Only Death Eaters with a Dark Mark on their arm could pass through the barrier unhindered. If the area around the barrier collapses, the barrier would be dispelled.

Severus Snape, though a loyal member of the Order of the Phoenix, was able to pass through because he had been branded with the Dark Mark.

Behind the scenes

Since the Dark Mark was invented by Lord Voldemort, it is likely that this curse, compatible with the Dark Mark, is also one of his inventions.

This curse (or a variation) could possibly have been placed on the gates of Malfoy Manor when Voldemort was using it as a hideout because Snape and Yaxley had to raise their left hand (where the Dark Mark was branded) in a salute to pass through it, while other who touch the gates would cause it to bend violently into a human-head shape.